The present invention relates to current pulse generator circuits, and in particular to circuits for the generation of current pulses of extremely short duration, which may be used in monolithically integrated devices using bipolar technology with separate power supplies.
The pulse generator circuits which are conventionally used in monolithically integrated devices generally comprise a capacitor whose charging and subsequent discharging are controlled by transistors which operate as switches, switching from a cut-off state to a conductive state and vice versa.
In the conductive state, the transistors operate at saturation in such a way that this state is in effect accurately defined.
A typical example of these pulse generator circuits is incorporated in the monolithically integrated device having the Part No. TDA 7272 produced by SGS Microelettronica S.p.A.
If the components of the circuit are normally dimensioned, current pulses having a duration in the microsecond (.mu.s) range are obtained.
The most recent embodiments of monolithically integrated circuits for use with telephones, for example, interface circuits between exchange components and subscribers' telephone lines, have led to the problem of inserting known pulse generator circuits in integrated devices having two separate supplies simultaneously, one for circuit components operating with voltages of approximately 50 V and the other for circuit components operating with low voltages of approximately 5 V.
Since in these devices the semiconductor substrate of the integrated circuit is biased to approximately -50 V with respect to a reference potential, the presence of transistors at saturation of the pulse generator circuits in circuit components supplied with a low voltage leads, despite the conventional isolation devices, to a considerable supply current absorption which is discharged through the substrate, thereby causing dissipation problems. Circuit and technological devices to prevent this loss of current through the substrate would entail a considerable increase in integration area occupation and a reduction of the speed of operation, i.e. the opposite of the requirements for such telephone circuits.